This invention relates to collision avoidance systems for vehicles such as aircraft, using the standard Aircraft Traffic Control Radar Beacon System (ATCRBS) signals to determine, at an Own station, the positions of Own and any Other transponder-equipped stations within the common areas of two or more secondary surveillance radar (SSR) stations.
Many collision avoidance systems using the ATCRBS signals have been devised or proposed. Some simply provide an indication or alarm upon proximity of Own and Other stations; some require active signal transmissions for determination of range; others require uplink data transmissions from ground-based equipment. All are subject, to a greater or lesser extent, to production of false alarms, or missed alarms or radio signal interference, such conditions occurring frequently under congested airspace conditions where such degradations are least tolerable. Determinations of bearings from Own to Other stations, desirable information, have heretofore been difficult to obtain; proposed airborne directional antenna systems for this purpose have proven too unreliable and costly to be practical. While North pulses can be used to determine bearings, this invention avoids the need for so-called North pulse kits to be installed on SSRs.